1. Field of the Invention
This invention relates generally to shredding devices, and more particularly to shredders which cut sheet materials in two transverse directions.
2. Description of the Related Art
Most paper shredders employ a pair of counter-rotating rollers having a plurality of interleaved cutting elements. The cutting elements generally conform to one of two categories, toothed discs and smooth-surfaced discs of right cylindrical configuration. Shredders employing toothed discs are typically constructed by attaching a plurality of discrete toothed discs and interspersed spacers to a shaft. Shredders employing smooth-surfaced discs are typically constructed by milling a piece of roll stock to form a plurality of spaced apart discs. The latter construction technique is preferable since the entire machining process is conducive to fully automated milling machines.
Both types of shredders function similarly. As shreddable material, such as paper, is fed between the counter-rotating rolls, the interleaved cutting elements cut or tear the material into longitudinal strips using a scissor-like action. U.S. Pat. No. 3,630,460 issued Dec. 28, 1971 to Goldhammer discloses a smooth-surfaced disc shredder having a plurality of interleaved, counter-rotating discs which cut sheet materials into strips using a scissor-like action. The teeth of the toothed discs or grooves in the smooth discs grip the material and pull it between the juxtaposed rolls to produce tension in the material which facilitates shredding. U.S. Pat. No. 3,033,064 issued May 8, 1962 to Lee discloses a shredder having a plurality of notched discs. The notches grip sheets of paper to advance them between the rollers where the interleaved, counter-rotating discs cut the paper into strips.
However, in many applications, such as governmental document destruction, this type of destruction proves inadequate. There is the possibility that the content of these waste documents can be reconstructed since characters remain on the strips. Therefore, each type of shredder has been improved to shred materials in both the longitudinal and lateral directions. U.S. Pat. No. 4,565,330 issued Jan. 21, 1986 to Katoh discloses a toothed disc shredder which uses teeth to draw the sheet materials between the shredding rolls. After the circumferential edges of the discs cut the material into strips, the teeth, in cooperation with a back plate, cut the strips into chips. U.S. Pat. No. 3,860,180 issued Jan. 14, 1975 to Goldhammer discloses a smooth-surfaced disc shredder having notches formed in the outer periphery of each disc such that the notches are disposed in a helical fashion along each roll. As the circumferential edges of the discs cut the sheet material into strips, the trailing edge of the notches cut the material strips into segments.
Although the above-mentioned techniques usually destroy documents satisfactorily, they demonstrate some inadequacies. Shredders similar to the Katoh shredder use "metal-to-metal" contact to cut strips into segments. This contact causes a significant amount of wear on the discs and rollers. Moreover, this segmenting technique produces relatively more stress between the rollers than do shredders similar to the Goldhammer shredder. Shredders, such as the Goldhammer shredder, must hold the sheet material very tautly in order for the sharp nose of the trailing edge of the notch to penetrate and cut the material into segments. If the material is loose or too thick, the nose of the notch will not be able to segment the strips.
Furthermore, both types of shredders cut paper into longitudinal strips using essentially the same technique. The circumferential edges of each type of disc form 90.degree. angles, and the interleaved discs produce a scissor-like action between the circumferential edges of adjacent discs. However, these edges are not sharp enough to cut through more than a few sheets of paper, and the cutting action relies heavily upon the tension or rigidity of the paper.
The present invention is directed to overcoming one or more of the problems set forth above.